The present invention relates to the field of communications and in particular to a communications system for the transport of ATM and AAL2-type signals.
The Universal Test and Operations PHY Interface for ATM (UTOPIA) bus defines an interface between the physical layer (PHY) and upper layer modules such as the ATM layer and various management entities in an ATM communications system. Two versions of UTOPIA bus have been defined by the ATM forum; UTOPIA level 1 is defined in “UTOPIA, an ATM-PHY Interface” specification level 1, version 2.01, Mar. 21 1994 (af-phy-0017.000) and UTOPIA level 2 is defined in af-phy-0039.000, June 1995. The UTOPIA bus structure supports ATM cells. UTOPIA level 2 also allows for shared interfaces that support many devices. UTOPIA comprises a data bus for carrying ATM cells and sufficient control information to control access to the interfaces. The control information may be carried via the data bus or using external control signals in parallel with the data bus.
The goal of the UTOPIA standard is to define a common, standard interface between ATM and PHY layers of ATM sub-systems. UTOPIA level 1 supports data rates up to 155 Mbit/s via an 8 bit data bus. UTOPIA level 2 supports data rates of up to 622 Mbit/s via an 8 or 16 bit data bus.
The International Telecommunication Union has defined in ITU-TI.363.2 B-ISDN ATM adaptation layer type 2 (AAL2). This specification includes definitions for so-called “AAL2 mini-cells” i.e. the common part sub-layer (CPS) packet. These AAL2 mini-cells are supported by ATM communication networks and currently a significant amount of traffic is carried in this form. The UTOPIA buses, as currently defined, are unable to carry AAL2 mini-cells. The arrangement of the present invention allows mini-cells and ATM cells to be carried via the same bus.